Process of tanning hides or skins



Patented 0a. 13, 1925.

UNITED STATES 1,557,011 PATENT orr cs.

JOHN REGINALD BLOCKEY, or BOLTON, ENGLAND, ASSIGNOB. *ro WILLIAM watxna AND sons LIMITED, or BQLTON, ENGLAND.

PROCESS OF TANNING HIDES 0R SKINS.

No Drawing.

To all whom it may concern:

Be it known that 1, JOHN REGINALD BL-ooKnY, a subject of the King of Great Britain, and resident of Rose Hill'lannery, Bolton, in the county of Lancaster, England, have invented a certain new and useful Improvement in the Processes of Tanning Hides or Skins, of which the following description is a specification.

In the tanning of hides or skins as at present carried out, it is diflicult to secure and retain that degree of acidity of the tanning liquor which is known to give the best and most constant results. Ordinary tan liquors prepared from infusion of raw tanning materials such as barks, are acid in property. The tanning agents themselves are acid and have by decomposition and fermentation during use other acids, such as acetic and lactic, added to them.

By the term acid is meant that the pH of the liquors is below 7. Most ordinary liquors have a pH of about 4 and the pH has a fundamental influence on the swelling of hides and skins. 7

It has been found that the maximum swelling occurs at about pH 2.7 on the acid side and at about pH 12 on the alkaline side, whereas a point of minimum swelling is about pH 4.7 acid.

In order to increase the swelling of the pelt or the leather, acids have been added to the tan liquor but under such conditions that unless carefully controlled and attended to erratic results are produced and often the leather turned out isdamaged.

The increased swelling in such cases is due to the'pH" of the liquor being reduced from about 4: to nearer 2.7 which is a point of maximum swelling.

' Another factor which influences the swellin is'the presenceof neutral salts.

dhe effect of neutral'salts upon the swellingis generally to reduce it when the solution i's acid as'all tan liquors are. If sufficient salts be present the whole of the swelling, which would otherwise be caused by the acidity oft-he liquor, can be pre- 'vented. For example, if the solution or liquor-be brought to pH :2 by the addition ofacid or otherwise, the: swelling will probablywbe excessive, but if,in additionl5% or more ofsodium chloride be added to the solution or liquor this swelling may be avoided.

Application filed December 16, 1924. Serial No. 756,355.

The swelling of thepelt when immersed in the tan liquors varies with variations in the acidity and salts present in the liquors. The acidity and the salts may be diminished or increased by the chemicals occurring in the hides to be immersed; further, the tanning materials used may contain salts, and the acidity, as before stated, may be altered by fermentation.

The effect of neutral salts on the swelling of pelt by acid during tanning-is a feature which heretofore has not been controlled.

The present invention consists in following such a process or method during tanning, that the above two factors are controlled to produce any desired degree of swelling or falling.

As the acids and neutral salts in ordinary tan liquor are uncertain both as to quantity and origin, in order to control the swelling or falling it is necessary to control the pH. This may be effected by the addition of acid or otherwise such as dilution or neutralization. It is also necessary to control the neutral salt effect by the addition of salts or otherwise, such as dilution.

One method of carrying out this process to obtain any desired swelling is tobring the tanning liquor to such a pH as .to produce a greater swelling than is required and to add to the liquid thus produced sufficient neutral salt to bring this swelling down to the exact point required.

By these means also it is possible to ha-ve the same conditions of swelling both before tanning and during the whole of the tanning processes.

Having determined what pH and which concentration of neutral salts are necessary to producethe required degree of swelling I have found it better for practical purposes not only to maintain these two factors in the tan liquors (in the manner already described) but also to treat the pelt before entering the tan liquors to an aqueous solution of acid and neutral salt possessing the same pH and the same concentration of neutral salt as the subsequent tan liquors, until the desired equilibrium isattained. In this waythe change of pelt from the alkaline condition to the acid condition is carried out apart from and not in the presence of tan and this prevents other undesirable" results. By, existing methods the pelt when grain defects and in the tan liquors has a different pH from that of the tan liquors and frequently pelt containing lime is placed in tan liquors. In any case by existing methods the placing of the pelt in the tan liquors causes wide fluctuations in both the pH and in the neutral salt content of the tan liquors, particularly so in the early tan liquors in the common system of suspender pits. This causes wide fluctuations in the swelling of the pelt in these tan liquors. Generally, the pelt, as it passes from the alkaline swollen condition (limed pelt) to the acid condition of the tan liquors, passes through a period or condition of fallenness. If this occurs in the presence of tan, grain defects and other undesirable results are produced.

By the present invention the pelt is according to one arrangement first brought to the same degree of swelling as is maintained during the tanning operation itself, and the degree of this swelling can be modified at will.

By bringing the tanning liquor to such a pH as to produce a much greater swelling than is required and by adding tothe liquor thus produced sufficient neutral salt to bring this swelling down to the exact point required, the great advantage is obtained, that small variations in the pH or in the salt content of the liquor due to extraneous causes do not influence the swelling to any appreciable extent. Thus at pH 2 without any addition of neutral salt the degree of swelling may be so excessive that the resulting leather will be commercially unsound (this frequently happens in the methods sometimes used to obtain increased swelling by adding sulphuric acid to the early tan liquors) but if neutral salt, such as sodium chloride be added to the extent of 2% the swelling may be brought to such a point that the leather Will be commercially sound. At the same time any variations in the pH or in the salt concentration due to extraneous causes already outlined, will not influence the swelling to any appreciable extent, whereas in processes heretofore followed these variations have produced uncertain and undesirable results.

In carrying out my process under manufacturing conditions I have found that the desired results may be obtained by preparing a liquor of acid, salt and water in the proportions of fifty pounds acid (if same is sulphuric acid) two hundred pounds of salt (which maybe common salt, NaGl) and one thousand gallons of water.

This liquor is placedin a pit or tank and the unhaired pelts are suspendedtherein' for not lessthan thirty six hours. It is found that the actions of the liquor. on the pelts :after this period havelittle if any further effect. To avoid waste of liquor and simp y the proced re I employ two pits r tanks which are supplied with 1i uor compounded as described. I then su ject the pelts or hides to the actions of said liquor in one pit or tank for a period of twenty four hours and afterwards remove said pelts or hides and place them directly into the liquor in the other pit or tank and leave same for a further period of twenty four hours.

On removing the pelts from the first of the pits or tanks above referred to I place the next batch or quantity of pelts or hides therein and into the same liquor from which the previous batch had been removed and I allow this freshly added batch to remain in the old liquor for twenty four hours. I then remove the said batch of pelts from their pit or tank and place them in the next or adjoining tank containing a newly or freshly compounded liquor and in this last liquor I again allow them to reinan for twenty four hours.

When I have removed the second batch of pelts 01' hides from the pit or tank after treatment therein, I run away or discharge the spent liquor from the said pit or tank andsupply same with freshly compounded liquor in order that same may be used for the succeeding batch. Thus I use each pit alternately and this may be during each twenty four hours.

After removing the pelts from their treatment in the acid liquor above described, 1 subject them to the ordinary process of tanning but with the tan liquor of the desired acidity that is pH 2 and with the same concentration of neutral salt as above stated.

To secure this desired acidity I may test (either by the hydrogen ion concentration measuring apparatus or by other well known process of testing) the tan liquor in each of the pits from the first wherein the most spent tan liquor is contained to the last pit containing the strongest tan liquor, and by the addition of acid and salt (that is to say if the liquor is too acid I dilute said liquor, whereas ifthe liquor is not sufficiently acid then I add the quantity of acid necessary to bring it to the required degree of acidity) bring the liquor to the desired pI-I. Or I may have one series of tan pits such as the usual system'of suspender pits supplied with tan liquor preparedto the exact degree of acidity by feeding such prepared liquor to the top or strongest pit. From this pit the tanning liquor flows as it becomes spent and asfresh liquors are added in the usual and well known manner until it is finally discharged in its fully spent condition, from the first pit in which the pelts are placed to commence their tanning process. Other systems of treating the'hides as by circulating the tan liquor or otherwise may be followed when using my method of control hereinbefore described. Y

I have found that when I prepare the tan liquor of the desired pH as last before described, the subjecting of the pelt to the tanning actions of same, after such pelts have been treated by my acid liquor, does not cause said tan liquor to change its pH to any extent that will interfere with the swelling of the pelt to the desired degree, thus I secure the controlling of said swelling by the treatment described.

To carry out the controlling of the swell-- ing during the period of actual tanning, that is to say, in the tan pits alone, the liquor in each pit is maintained at the desired pH and must contain the desired proportion of neutral salt usually approximately two percentum by weight.

When using solutions prepared by the proportions of acid and salt as hereinbefore described the pelt will be more swollen than is customary in natural tan liquors (that is, tan liquors which have had no acid added to them) and the degree of swelling will be approximately the same as is possessed by the pelt immediately after the usual methods of liming. If greater swelling is desired, this can be achieved by maintaining the same pH but by reducing the amount of sodium chloride. Conversely if less swelling is desired, this can be achieved by maintaining the same pH but by increasing the amount of sodium chloride. It is obvious that the details given may be varied in many .directions.

By adjusting the pH and the neutral salt concentration, any desired degree of swelling or falling can be produced and maintained throughout the whole tanning proc- It is well known that in the process of tanning, acid may be used for producing swelling, it is also well known that neutral salt will reduce swelling and that by taking an excess of sodium chloride (i. e. more than 5%) it will entirely prevent or completely reduce all swelling. Further it is common to useboth acids and neutral salts in the treatment of hides, but I have hereinbelore stated, by following my method of treatment I control the degree of swelling so that I can with the greatest precision and at all times swell pelt to the precise extent intended.

Such being the nature and object of my said invention, what I claim is l. A process of tanning hides and skins comprising preliminarily subjecting the pelts to the action of a solution, the acid strength 'of which is maintained substantially the same as that of the tanning liquor by addition of a properly proportioned amount of neutral salt, whereby the extent or degree of swelling of the pelts may be regulated as desired.

2. In the process of tanning hides and skins, treating the pelt, before entering the tan liquor to an aqueous solution of acid and neutral salt possessing the same pH and the same concentration of neutral salt as the tan liquor to the action of which it is subsequently submitted substantially as and for the purpose herein specified.

3. In carrying out the process of tanning hides and skin as herein described, bringing the tanning liquor to such a pH as will produce a much greater swelling than is required and adding thereto sufiicient neutral salt to bring this swelling down to the point required, substantially as herein specified.

JOHN REGINALD BLOCKEY. 

